(a) Technical Field
The present invention relates to a system and method for controlling creep torque of an electric vehicle. More particularly, it relates to a method for controlling creep torque, which can efficiently prevent initial slope rolling-back of a vehicle using an electric motor as a driving source.
(b) Background Art
Today, vehicles using fossil fuel-combusting gasoline and diesel engines have various environmental contamination limitations due to exhaust gases, global warming due to CO2, respiratory ailments due to ozone generation, and fuel resource depletion.
In order to help prevent these effects, eco-friendly electric vehicles including pure electric vehicles (EV) (vehicles using a battery as a power source and a driving motor as a driving source), hybrid electric vehicles (HEV) (vehicles using an engine and a driving motor as driving sources), and fuel cell electric vehicles (FCEV) (vehicles using a fuel cell as a power source and a driving motor as a driving source) have been developed.
In a typical gasoline engine vehicle, since an idle torque of an engine is delivered to a torque converter and a transmitter even when an accelerator pedal or a brake pedal is not depressed, the vehicle can creep forward at a low speed even when the driver is not pressing the accelerator pedal.
As shown in FIG. 5, a vehicle ascending a slope undergoes a slope resistance of mg sin θ as well as a rolling resistance. Due to the slope resistance, the vehicle may stop at the slope by a frictional braking force according to the operation of a brake pedal. Thereafter, if a driver releases the brake pedal, the vehicle may in this instance move backward. In this case, the driver may prevent the vehicle from rolling backwards by stepping on the brake pedal or the accelerator pedal.
In order to avoid performing such operations for preventing a vehicle from rolling backward on an upward slope, creep control technologies have been introduced. In these cases, creep control technologies may prevent the rolling-back of a vehicle even when an accelerator pedal or a brake pedal is not depressed, and may switch the vehicle into a normal driving state when a driver steps on the accelerator pedal. This is done by allowing the idle torque of an engine to be delivered to a torque converter and a transmitter when it is detected that the vehicle is on a hill.
Since electric vehicles operate using only a motor driving system (i.e., a motor, a decelerator, and a wheel) without a separate engine, idle torque from the motor does not automatically occur when the electric vehicle stops. Accordingly, creeping like in a gasoline engine vehicle can not occur.
Accordingly, a creep function to simulate creeping (i.e., like those vehicles having a gasoline engine) has to be implemented in electric vehicles through motor torque control.
In other words, pure electric vehicles, including fuel cell vehicles, require a creep control algorithm for generating a torque similar to an engine idle torque to provide a driving feeling similar to that of typical gasoline vehicles. Many studies are being conducted to develop creep control algorithms for improving a driving feeling and preventing a vehicle from rolling back when it restarts from a halt state on an upward slope.
For example, in a vehicle driven by only a motor like an electric vehicle or a fuel cell vehicle, rolling-back on an upward slope may be prevented by controlling a motor driving force without an additional system (e.g., by recognizing an upward slope and increasing a motor driving torque).
That is, as shown in FIG. 6, fuel cell or electric vehicles may determine an upward slope using a slope angle sensor (G-sensor) or a rolling-back determination algorithm, and may increase motor torque according to the above determination result. However, in a typical creep torque control method for preventing a vehicle from rolling back on an upward slope, the vehicle frequently moves back or suddenly rushes forward because of various factors, and thus there is much room for improvement in terms of sensitivity. Also, the method is limited in that a countermeasure against malfunction of a sensor is not been addressed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.